changegroup: make some packer attributes private
These methods and attributes are low level and should not be
called or outside outside of instances. Indicate as such through
naming.
Differential Revision: https://phab.mercurial-scm.org/D4085
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}